HIGHLIGHT: A new psychology says that the mind is not a
computer that works by the rules of logic, but a set of tools evolved to
help people live pre-industrial lives

YOU are a barman and you will lose your licence if
you serve a drink to an underage drinker. At your bar are four people; you
know what two are drinking (one has beer, one has coke) and you know the
ages of the other two (one adult, one teenager). Ask the minimum number of
questions that will ascertain if you are breaking the law. If your answer
is that you need only ask the beer drinker's age and teenager's tipple,
then you join the 75% of those asked the question who get it right. Muted
congratulations.

Now consider someone faced with cards which have letters on
one side and numbers on the other. He wants to check the rule "a card with
a D on one side must have a 3 on the other", and he is presented with card
D, card F, card 3 and card 7. Which cards must he turn over? More
congratulations for the right answer this time, because only 25% of people
say D and 7.

The intriguing thing about these
two problems is that, to a logician, they are the same. The structure of
the card problem, and the answer, are identical to the drinking problem.
Why then is one problem easy and the other relatively hard? A small group
of psychologists think they know the answer to this meta-question; if they
are right, a new theory of the mind will be in order, one which has no
such thing as general intelligence within it, and is not dominated by
symbolic reasoning skills. The mind is not, they say, a reasoning machine
-- it is a machine designed for scraping out an existence in a clan of
hunter gathers.

The tests in the first paragraph are called Wason tests,
after the psychologist who first tortured people with them. They are the
essence of a simple reasoning task -- the application of the rule "if p,
then q". If the mind were a straightforward reasoning machine, all Wason
tests would be equally soluble. In fact their solution depends on the
story around them. Psychologists first guessed that it is all about
familiarity story would be easier for the mind to reduce to soluble ps and
qs. The barman's problem is obvious, the other one is obscure. But
experiments have ruled that out. Familiar contexts ("if a person eats hot
chilli peppers, then he will drink cold beer") prove difficult, while
strange ones ("if a man eats cassava root, then he must have a tattoo on
his face") sometimes prove easy.

Leda Cosmides and John Tooby of the University of
California at Santa Barbara are among the band that thinks it has an
explanation. They argue that the underlying logical structure is of little
relevance, and that the familiarity of the context does not matter much
either. What matters a lots is the nature of the context. If a rule of the
form "he who takes the benefit must pay the cost" is at stake, then
solving the problem means spotting cheats. People do this well. The mind
is not following abstract reason; it is enforcing a social contract.

To demonstrate this, Dr Cosmides gave students
at Stanford University a series of Wason tests. Some were set in a
fictitious culture in which rules such as the one that restricts cassava
root to tattoo-wearers are laid down by a chief called Big Kiku. Others
were simply nonsensical conjunctions of events: "If you eat duiker meat,
then you have found an ostrich eggshell". The students proved far better
at enforcing Big Kiku's laws than at pursuing arbitrary pieces of if-then
logic.

Gerd Gigerenzer, of Salzburg University, and his colleagues
have gone one step further. In an ingenious Wason test, they asked two
groups of students to turn over cards to test the statement: "If an
employee gets a pension, then that employee must have worked for the firm
for at least ten years." "Worked for eight years", "Worked for 12 years".
The difference between the two groups was that one was told they were
employers, the other that they were employees.

If they were solving the problem in some purely logical
way, both groups should get the right answer; the rule is broken only when
somebody has worked for less than ten years but gets a pension, so the
cards to turn are "Gets pension" and "Worked for eight years". But if they
are looking for cheats, employees will worry about those who worked for 12
years and do not get a pension, even though this is strictly irrelevant to
the problem. So it proves. Almost all the employers, whose interests
coincide with the right answer, turned the correct cards. The employees,
however, apparently more concerned with justice than logic, plumped for
"Did not get a pension" and "Worked for 12 years" by a ratio of six to
one.

Dr Cosmides and Dr Tooby take all this to mean
that the Wason test awakens a specific mental mechanism that keeps the
accounts in social contracts and is on the lookout for cheats. Following
on from that, they suggest that the brain is a bundle of such job-specific
mechanisms, shaped by evolution, rather than applying the same
general-purpose4 "reason" to all the problems it encounters.

The inspiration for this notion is the idea that society is
based on social exchange of the form "You scratch my back, I'll scratch
yours." In animal societies, all apparently altruistic behaviour that is
not based on kinship seems to work like this. Baboons help each other in
fights and keep a close account of who owes whom favours. A vampire bat
the does not regurgitate part of its blood meal with a neighbour who came
home hungry forfeits the return favour at a later date.

Some anthropologists are coming to see human societies in
much the same light. Kim Hill of the University of Michigan, who studies
the Ache in Paraguay, has found that a hunter who returns from the forest
laden with meat will give some to his partner and children, some
surreptitiously to a woman he wants to have sex with -- the trade is
explicit -- and some to other hunters who might return the favour later. A
fatherless Ache family often nearly starves, because nobody has an
incentive to share meat with a family that cannot reciprocate. Such a
system of debts is well suited to hunters. A hunter may return
empty-handed for days on end and then suddenly kill a tapir -- far more
than he can eat. Better to share it, and thus be owed a debt, than waste
it.

Gambling on certainty

Other aspects of rationality are starting to fall by the
wayside, too. Dr Gigerenzer, a probability theorist by training, has
tried, using similar ideas, and similar experiments to his work on social
contracts, to explain the mistakes people make when thinking about
probabilities.

Probability theorists have always been split over the
question of what probability is. "Bayesians" -- named after the originator
of their point of view -- say it is a measure of subjective certainty
about single events: "I'm 90% sure of my horse winning this race". "Frequentists" say it is the long-run
frequency of events: "Nine out of my last ten tips were winners". People
are quite good at assessing the latter while, to the delight of
bookmakers, they are generally hopeless at the former.

One example of this is the psychological paradox known as
the "overconfidence effect". Overconfidence tends to be specific rather
than general. When asked a general-knowledge question such as "which city
is bigger, Bonn of Heidelberg?" people are more likely to think they know
the correct answer than actually to know it. But after answering a string
of questions, they are good at estimating the number they got right.
Psychologists have used such "fallacies" to argue that people are bad at
statistics. Dr Gigerenzer thinks, rather, that people are natural
frequentists. He has found that merely rephrasing a problem in frequentist
rather than Bayesian terms generally increases the number of people who
can solve it (see box).

Again, a look at primitive life suggests a reason why. The
probability of a single event is a meaningless fact in a hunter's world:
what can he do about the fact that his chances of killing a tapir today
are 3%? But the frequency of past events and past conjunctions is vitally
important and always has been: he killed a tapir on three of the last 100
visits to this one. A German psychologist, Egon Brunswik, argued as early
as the 1960s that human brains are constantly, and unconsciously,
assessing such frequencies as guides to future events.

Given this view of man -- a natural trader, ever concerned
with social debts and an uncertain future -- it is little wonder that
human minds are interested in detecting cheats, not pursuing pure logic,
and in sampling frequencies rather than making risky one-off guesses.
Reasoning, in this view, depends on a number of such mental subroutines.
Logic is a refinement and codification of their results -- a creative and
powerful generalisation, but the crowning glory of human intellectual
achievement rather than its deep foundation.